As a background of explosive diffusion of smartphones, portable tablet terminals, and the like and the start of video distribution services, demand for increase in optical network transmission capacity continues to grow day by day. Further development of optical communication technology is required by responding to this demand, and techniques for realizing downsizing and cost reduction for components used in optical communication systems are increasingly important. As one of the techniques that has been playing an important role to implement the components for optical communication systems, there is a waveguide type device. In the waveguide type device, various fundamental functions such as an optical signal branching/coupling device, a wavelength multiplexer/demultiplexer, an interleaving filter, an optical switch, and a Variable Optical Attenuator (VOA) are implemented by applying the interference principle of a light. As these devices have the structure of a waveguide type, they have features such that their circuit designs are flexible and easy for the size increase and high integration. Further, waveguide type devices are manufactured by using a process of manufacturing semiconductor components such as an LSI, and thus, they are also highly expected as devices having excellent mass productivity. As a material for a waveguide part, various materials such as semiconductors and polymeric materials are put into use. Particularly, a silica-based optical waveguide fabricated on a silicon substrate has features of excellent stability achieving low loss and excellent matching with optical fibers, and further, it is one of the most practical waveguide type devices.
In order to respond to the demand for increasing the above-described optical network transmission capacity, digital coherent optical transmission technology has been widespread. Among optical communication components configured by using the waveguide type device, an optical transceiver used for digital coherent optical transmission is particularly focused on. This optical transceiver implements, in a Wavelength Division Multiplexing (WDM) optical signal, high-speed operation with the transmission rate of 100 Gb/s per wavelength.
An optical signal modulation scheme mainly used in the digital coherent optical transmission technology is a phase modulation. Specifically, Phase Shift Keying (PSK) or Quadrature Amplitude Modulation (QAM) scheme, which is a phase modulation scheme which combines with intensity modulation, is used. Furthermore, in the digital coherent optical transmission technology, in addition to the phase modulation, a polarization multiplexing scheme that multiplexes a plurality of phase-modulated optical signals by two orthogonal light polarizations is combined to implement the above-described high-speed transmission rate.
An optical receiver in a digital coherent optical transmission system includes an optical interference circuit which performs, at its front end, signal processing for an optical signal. Interfering light obtained from the optical interference circuit is detected by Photo Detector (PD) to convert it into an electric signal, and a received signal is obtained. The received signal from the optical interference circuit implements, via subsequent digital signal processing, modulation of a polarization-multiplexed phase modulation signal.
In the above-described optical interference circuit, the optical waveguide type device is widely used, and includes fundamental elements such as a VOA which adjusts the light intensity of a signal light, Polarization Beam Splitter (PBS) which splits polarized waves of a signal light, a polarization rotator (polarization rotating device) which rotates a polarized wave of a signal light or a local light, and a 90-degree hybrid which detects waves of a retardation by interference between the signal light and the local light. Particularly, an optical waveguide type device using the silica-based optical waveguide is also generally called a Planar Lightwave Circuit (PLC). In realizing further diffusion of digital coherent optical transmission systems and the increase in their capacities in future, the optical receiver including the PLC will be a key component.
FIG. 1 is a diagram showing a configuration of an optical interference circuit configured by the PLC in an optical receiver of a conventional technique, and is a top view viewing a substrate face of a silicon substrate on which an optical interference circuit is configured. Here, an explanation on detailed operation will be omitted, but the diagram depicts substantial shapes of the practical fundamental elements in the optical waveguide type device for implementing different functions of the optical interference circuit. An optical interference circuit 100 also includes, as major fundamental elements, a VOA 15, a PBS 12, a polarization rotator 13, and 90-degree hybrids 16a, 16b. Further, it also includes a signal light input waveguide 11, a local light input waveguide 14, interfering light output waveguides 18a, 18b, and a signal light monitor waveguide 17. In the optical interference circuit configured by the PLC and including combinations of each of fundamental elements to implement different functions as shown in FIG. 1, the downsizing is an extremely important technical problem.
In implementing the PBS or the polarization rotator in the PLC, a configuration of inserting an optical retardation plate into the optical interference circuit so as to intersect an optical waveguide may be used. The optical retardation plate is an element that causes retardation to an optical signal depending on the polarized wave of the optical signal passing through the optical retardation plate, and further, the optical retardation plate fabricated by using, for example, a polyimide film is widely known.
With reference to FIG. 1 again, in the PBS 12 and the polarization rotator 13, grooves 3 for inserting the optical retardation plates are formed in a manner of intersecting the optical waveguides. The optical retardation plate is inserted into this groove 3 so that light propagating via each of the optical waveguides passes through the optical retardation plate. Due to the configuration including such a groove on the substrate face, it is possible to cause rotation for the light polarization that passes through the optical retardation plate. For instance, in order to configure the PBS 12, a Mach-Zehnder optical interference circuit configured by two optical waveguides may be used for inserting a λ/4 wave retarder into each of the optical waveguides such that their birefringent optical axes are orthogonal to each other. In addition, in order to implement the polarization rotator 13, a λ/2 wave retarder may be inserted such that the birefringent optical axis is in 45-degree direction with respect to a targeted optical waveguide (Non Patent Literature 1).